Exposure compensation was a useful feature for better previewing
colors when scene exposure was not neutral. It was removed in
4bf15a06f3 - [D15350](https://archive.blender.org/developer/D15350)
after any socket could be connected to the shader output.
This commit reintroduce this feature, but makes it disabled by default
as this workflow is not always wanted. In the case of an NPR scene
where every material is emissive so there are no actual lighting
calculations, the user may want to use the preview feature to quickly
select a texture without having it exposure-compensated.
